Delivering a colourant such as astaxanthin with good oral bioavailability for improved plasma uptake and flesh deposition in salmonid is a particular concern for fish feed producers and fish farmers. Because of poor solubility in a physiological milieu, administration of astaxanthin crystals in fish feed pellets and other nutritional dosage forms does not give sufficient oral uptake of the colourant. In order to make the colourant more bioavailable, several methods have been developed for preparing particulate astaxanthin compositions which are dispersible in water for processing into feed pellets. The dispersible compositions are prepared by dissolving crystalline astaxanthin in solvents (U.S. Pat. No. 6,863,914 and U.S. Pat. No. 6,406,735) or oils (U.S. Pat. No. 5,364,563) under high pressure and temperature, immediately followed by dispersing the organic solution in aqueous hydrocolloid. Alternatively, the carotenoid is melted in an aqueous excipient-matrix and emulsified under pressure without using solvent or oil (U.S. Pat. No. 6,093,348). All the methods involve further processing to prepare powder formulations from the aqueous dispersions. None of the disclosures describe the type of astaxanthin crystal used in terms of their crystal structure as disclosed by X-ray diffraction and Raman spectroscopy. Despite the onerous conditions required for preparing astaxanthin compositions there has been little effort directed towards making the methods more production friendly. One way to reduce energy and solvent consumption would be to utilise different crystal forms of astaxanthin that have advantageous solubility, melting or stability characteristics, thereby allowing more gentle processing conditions. Surprisingly, the disclosures are particularly silent on specific crystal forms of astaxanthin and their potential utility for preparing astaxanthin compositions. Different crystal forms can affect in vivo dissolution rate and allow higher (supersaturated) concentrations of carotenoids in oily administration vehicles which may in turn provide higher oral uptake and bioavailability.
U.S. Pat. No. 6,827,941 describes amorphous aggregates of astaxanthin which are prepared from dilute astaxanthin solutions in acetone (50 mg/liter) followed by 20 fold dilution with water/acetone 7/3 (v/v.). It is silent regarding formation of specific crystal forms. U.S. Pat. No. 5,654,488 describes the synthesis and crystallisation of trans-astaxanthin from the reaction mixture using the Wittig process but fails to specify the crystal structure or structures that are obtained. The disclosures of the foregoing patents are incorporated herein by reference in their entireties.